mlog: refactor From to include Component's Context implicitly

mlog/cmp.go

@@ -2,6 +2,7 @@ package mlog
 
 import (
 	"github.com/mediocregopher/mediocre-go-lib/mcmp"
+	"github.com/mediocregopher/mediocre-go-lib/mctx"
 )
 
 type cmpKey int
@@ -19,10 +20,29 @@ var DefaultLogger = NewLogger()
 // From returns the Logger which was set on the Component, or one of its
 // ancestors, using SetLogger. If no Logger was ever set then DefaultLogger is
 // returned.
+//
+// The returned Logger will be modified such that it will implicitly merge the
+// Contexts of any Message into the given Component's Context.
 func From(cmp *mcmp.Component) *Logger {
-	l, _ := cmp.Value(cmpKey(0)).(*Logger)
-	if l == nil {
+	var l *Logger
+	if l, _ = cmp.Value(cmpKey(1)).(*Logger); l != nil {
+		return l
+	} else if lInt, ok := cmp.InheritedValue(cmpKey(0)); ok {
+		l = lInt.(*Logger)
+	} else {
 		l = DefaultLogger
 	}
+
+	// if we're here it means a modified Logger wasn't set on this particular
+	// Component, and therefore the current one must be modified.
+	l = l.Clone()
+	oldHandler := l.Handler()
+	l.SetHandler(func(msg Message) error {
+		ctx := mctx.MergeAnnotationsInto(cmp.Context(), msg.Contexts...)
+		msg.Contexts = append(msg.Contexts[:0], ctx)
+		return oldHandler(msg)
+	})
+	cmp.SetValue(cmpKey(1), l)
+
 	return l
 }